Document FR-A-2 659 546 discloses a spinal osteosynthesis system which comprises a collection of pedicle screws, each having a spherical seat made in a so-called "forked" head and in which a split ring can be housed. Passing through this ring itself is a rod intended to connect the various screws together, and the ring offers a possibility of angularly orientating the axis of the pedicle screw and the axis of the rod in one or two planes. Once this angular adjustment has been performed, a threaded cap compresses the ring and locks the assembly firmly in position.
The known ring is made of one single piece, with a spherical outer face, a cylindrical inner face with symmetry of revolution, and a slot directed radially and extending between the inner and outer faces.
This slot gives the ring the required deformability so that when clamped into the seat in the pedicle screw using an appropriate threaded member, it can be compressed firmly against the rod, so that once the angular adjustment has been performed, the assembly is held perfectly immobilized.
This known ring does, however, have a limitation: when preparing the implant system prior to or during fitting, it is necessary to slip as many split rings as are needed over the rod from one of its ends.
This is because the deformability of the ring which is afforded by its slot is far from sufficient to allow this slot to be parted temporarily to allow the rod to be introduced radially into the cylindrical passage through the ring. Thus, the practitioner attempting to proceed in this manner would inevitably cause breakage or significant damage to the mechanical qualities or the shape of the ring.
The use of this kind of known ring is therefore restricted to scenarios in which the rod is accessible from at least one of its ends.